1. Field of the Invention
This invention relates to Q-switched lasers and more specifically to the use of a shared modulator to control a plurality of Q-switched fiber lasers to provide temporally overlapping pulses. A pair of Q-switched fiber lasers at slightly different wavelengths being well suited to provide a low-frequency light source through difference frequency generation (DFG) by nonlinear optical materials.
2. Description of the Related Art
Q-switching is a widely used laser technique in which a laser pumping process is allowed to build up a much larger than usual population inversion inside a laser cavity, while keeping the cavity itself from oscillating by removing the cavity feedback or greatly increasing the cavity losses. After a large inversion has been developed, the high Q cavity is restored to its usual large value, hence bringing the Q-factor to a high value, producing a very short, intense burst which dumps all the accumulated population inversion in a single short laser pulse. Modulation of the cavity produces repetitive pulses.
As illustrated in FIGS. 1a through 1d, the cavity loss 10 is initially set at some artificially high value—that is, at an artificially low value of the laser cavity Qc—while the inversion 12, hence the gain and the stored energy, in the laser medium are pumped up to a value much larger than normally present in the oscillating laser. The cavity loss is then suddenly lowered to a more normal value (higher Qc) so that the round-trip gain after switching is much larger than the cavity loss. The initial stimulated emission in the laser cavity then immediately begins to build up at an unusually rapid rate, soon developing into a rapidly rising and intense burst, or “giant pulse” 14 of laser oscillation. The oscillation signal rapidly drives the inversion 12 down below the new cavity loss level, after which the oscillation signal in the cavity dies out nearly as rapid as it rose. The peak power in the Q-switched pulse can be several orders of magnitude more intense than a cw laser created in the same laser with the same pumping rate.
Some of the more common free-space Q-switching methods employed in practical laser systems are shown in FIGS. 2a through 2c. The laser cavity includes a gain medium 22 and a pair of mirrors 24 and 26. As shown in FIG. 2a, mirror 26 is rotated (spinning motor shaft) so that the laser is oscillated only during the brief interval when mirror 26 is aligned with mirror 24. As shown in FIG. 2b, an electrooptic crystal 28 which becomes birefringent under the influence of applied voltage and one or more prisms 30 are placed inside the cavity. The birefringent crystal 28 rotates the polarization of the light energy so that it is coupled out of the cavity by the prism 30. Electrooptic Q-switching provides fast switching with precise timing and good stability but the repeat rate is relatively slow and the crystal and pulse source are fairly expensive, as the voltage needed to switch one polarization to the other is more than a few hundred volts. As shown in FIG. 2c, an rf acoustic wave created in the optical material 32 at the Bragg condition diffracts light out of the cavity to lower the Q. Acoustic modulators have a fast repeat rate but are expensive and a slower switch speed as well as limited aperture size.
Cai et al. Optical Fiber Communication Conference and Exhibit, 2002, 17-22 Mar pp 654-655 reports on a Q-switched erbium doped fiber laser using a fiber Bragg grating placed in a loop mirror (FBGLM) as an all-fiber wavelength-selective intensity modulator. Cai's FBGLM acts like a Michelson interferometer in which Q-switching is achieved with a PZT that stretches/compresses the fiber axially to change the optical path length between the 3 dB-coupler and the FBG in the upper portion of the loop. A detailed explanation of the FBGLM is provided in Zhao et al. “A New Structure Optical Fiber Wavelength-Selective Switch”, CLEO, Pacific Rim 1999, pp. 525-526.
In many cases, for example, optical nonlinear processes in nonlinear crystal such as difference-frequency generation (DFG), two Q-switched lasers with excellent temporal overlap are needed. For GHz or THz generation two laser beams with a specific frequency difference will be needed. Since the laser pulse is on the scale of nanoseconds, the two pulsed beams have to be controlled extremely carefully, which is very difficult to achieve.
Hatanaka et al. “Tunable terahertz-wave generation from DAST crystal by dual signal-wave parametric oscillation of periodically poled lithium niobate”, Optics Letters, Vol. 25, No. 23, Dec. 1, 2000, pp. 1714-1716 describes a THz source that uses a single free-spaced Q-switched Nd:YAG laser to pump a cavity having two different wavelength grating to produces pulses at two different wavelengths, which are input to a DAST to produce a THZ source through difference frequency generation. This structure produces two perfectly overlapping pulses. However, the polarization of the beams should be perpendicular when they enter the DAST and controlling the polarization of two beams in one fiber, particularly a gain fiber, is more complicated.
There remains a need for generating simultaneous Q-switched fiber lasers that provides for narrow pulse widths and fast repeat rates.